Closed caption broadcasting is a broadcast service for separately transmitting text data, which has not been integrated with a television image, to allow a television receiver to selectively display the text as a caption. In an NTSC standard-based analog TV broadcast, caption data is transmitted by two bytes each time as analog waveforms at the 21st line of each of first and second fields of a vertical blanking interval (VBI), so that a receiver can display the caption data on its screen. Meanwhile, in ATSC type digital television broadcasting, closed caption data may be inserted by two bytes each time as a sort of user data within an extension_and_user_data( ) structure of a picture header, and control information regarding reproduction of the closed caption may be defined in an event information table (EIT) of a PSIP (See “ATSC Digital Television Standard Part 4—MPEG-2 Video System Characteristics (A/53, Part 4:2007), with Amendment No. 1” and “Digital Television (DTV) Closed Captioning, CEA-708-B”). The receiver may interpret the caption data received through the picture header according to the CEA-708-B standard and display the closed caption according to a definition or stipulation, which was made by a transmitter side, while accumulating the pair of bytes of the caption data.
A DTV closed caption (DTVCC) in conformity with the CEA-708-B standard will now be described briefly with reference to FIGS. 1 to 5.
DTVCC data is transmitted according to a particular type of data communication protocol. FIG. 1 shows a protocol stack of the DTVCC. The protocol stack includes five layers, namely, a transport layer, a packet layer, a service layer, a coding layer, and an interpretation layer.
The transport layer is a part in which the DTVCC data is inserted to a DTV video sub-system and extracted by the receiver, which follows the definition of the ATSC A/53 standard and ISO/IEC 13818. In detail, the DTVCC data is divided by two bytes each time and transmitted through picture user data of a DTV stream, and a descriptor for signaling is transmitted through a PMP and/or the EIT of the PSIP.
FIG. 2 illustrates a syntax of the closed caption data cc_data( ) transmitted through the picture user data. “process_cc_data_flag” is a flag indicating whether closed caption data cc_data( ) needs to be processed. That is, if the flag is set as ‘1’, the closed caption data should be parsed and processed, whereas if the flag is set as ‘0’, the closed caption data may be disregarded. “cc_count” field indicates the number of subsequent caption data structures and may have a value from 0 to 31. With respect to a certain number of caption data structures of the cc_count, 16-bit caption data is transmitted through a pair of “cc_data_1” and “cc_data_2” fields. When a “cc_valid” field is set as 1, it indicates that a subsequent 2-byte caption pair is valid, and when the “cc_valid” field is set as 0, it indicates that the subsequent 2-byte caption pair is invalid. A “cc_type” field indicates, for example, a type of the subsequent two-byte caption data a defined in the CEA-708-B standard. For example, if the “cc_type” field has a value of 10, it indicates that the 2-byte closed caption data is closed caption data, and if the “cc_type” field has a value of 11, it indicates a start of new closed caption data.
With reference back to FIG. 1, before the DTVCC data is coded in the transport layer, it is packetized into a caption channel according to a certain rule. The packet layer is defined by the caption channel packet. FIG. 3 shows the structure of the caption channel packet. An n-byte caption channel packet includes a 1-byte packet header and (n−1)-byte packet header. A caption header includes a sequence number and packet size data. The sequence number is 3-byte data determined by being circulated within the range of 0 to 3 in order to determine whether or not a packet has been lost.
The DTVCC caption channel may be divided into sets of logical sub-channels called a “service”. Service data is inserted into a caption channel data stream according to a time division multiplexing scheme. The service layer defines a header with respect to the number of closed caption data channel services, a service type, a service attribute, and the like. Six standard services and maximum 57 extended services can be added to the caption channel data stream, so a total 63 services can be provided. FIG. 4 shows the structure of a service block with respect to each service. A service block includes a service block header and service block data. The service block header includes a service number and service block size data. If the number of services is 6 or smaller, the service block header has one byte, while if the number of services exceeds 6, the service block header has two bytes.
With reference back to FIG. 1, the coding layer describes how data is coded for a closed caption service. Namely, the coding layer defines allocation of code space control, a caption command, and numeric codes for caption characters and symbols. FIG. 5 shows the configuration of a code space according to the CEA-708-B standard. The code space includes two code books each having 256 positions, which are divided into four code groups: CL, GL, CR, and GR. The CL group includes 32 codes from 00h to 1Fh, to which a C0 code set (various control codes), and a C2 code set (various extended control codes) are mapped. The GL group includes 96 codes from 20h to 7Fh, to which a G0 code set (a character set) and a G2 code set (extended control code 1) are mapped. The CR group includes 32 codes from 80h to 9Fh, to which a C1 code set (caption control code) and a C3 code set (extended control code 2) are mapped. The GR group includes 96 codes from A0h to FFh, to which a G1 code set (Latin characters) and a G3 code set (characters to be used in the future/icon extended set) are mapped.
Basic codes of the four code groups (CL, GL, CR, and GR) are characters, control codes, and commands of C0, C1, G0 and G1 code sets at an upper portion in FIG. 5. C2, C3, G2, and G3 code sets at a lower portion in FIG. 5 are approached by using an ‘EXT1’ code (10h) of the C0 code set. Namely, by adding the ‘EXT1’ code in front of a code or a symbol within a code space, extended C2, C3, G2, and G3 code sets can be referred to. In other words, in order to refer to the characters present in the C2, C3, G2, and G3 code sets, two bytes (i.e., ‘EXT1’+basic code) must be used.
The interpretation layer defines how caption data is encoded by using the DTVCC graphic user interface, namely, the code sets of the coding layer, and how the caption data is interpreted when decoded. The interpretation layer handles a caption screen, window, pen, text, and display synchronization.
According to closed caption broadcasting implemented by those protocols, the lines (i.e., speech, dialog) of the players, lyrics of songs, movie lines translation, online TV guide, an emergency broadcast, and various other molecular services can be provided. Recently, as closed caption broadcasting tends to become mandatory limitedly in terms of media access rights of the hearing impaired or a comprehensive service, the closed caption broadcasting is anticipated to extend in its utilization.
Meanwhile, the advancement of television technology has reached a level of implementing a device for displaying stereoscopic images (or three-dimensional (3D) images), and in particular, a full-scale commercialization of a stereoscopic type 3D television is around the corner at the time of filing application of the present invention. In a stereoscopic 3D display system, two images are taken by using two image sensors spaced apart by about 65 millimeters like the human's eyes, which are then transmitted as broadcast signals to a receiver. Then, the receiver allows the two images to be inputted to the left and right eyes of a person (i.e., a user), thus simulating a binocular disparity to allow for a deep perception or stereoscopic view.
In this manner, when closed captions are intended to be implemented in the stereoscopic type 3D television, it is desirous for caption text to be stereoscopically styled and displayed. A scheme in which the receiver itself renders the 3D caption image based on the caption text, but real time defining 3D attributes (e.g., the thickness and cubic (solid) color of the caption text, a color and transparency of the caption text display region, etc.) with respect to a large quantity of continuously inputted caption text and 3D rendering the same may overly increase the calculation burden of the receiver. If 3D attributes to be indiscriminately applied to caption text are previously set and caption text is 3D-rendered according to the fixed 3D attributes, the calculation burden can be somewhat reduced, but the aesthetic sense of the 3D caption would be possibly greatly degraded and cause the user to become tired of it, significantly dampening the utility and charm of the closed caption.
Thus, a method for highly maintaining an aesthetic sense and charm of a caption image, while displaying a closed caption with a cubic effect so as to be consistent with (in harmony with) a 3D image displayed on a 3D television is required.